This invention relates to a thermal energy storage system. The invention is especially useful in storing heat for a solar energy space-heating system.
The present invention is an improvement in heat storage systems over the heat storage means disclosed in the following patents:
______________________________________ U.S. Pat. No. Patentee ______________________________________ 2,677,664 Telkes 3,464,486 Rice, et al 3,501,261 Rice, et al 3,743,782 Laing 3,884,295 Laing, et al ______________________________________
Solar energy heating systems, such as those used for space heating of buildings, have two major components--the solar heat collector and the heat storage system. The present invention is concerned with the heat storage system and the solution to problems which have plagued previous heat storage systems.
At the present time, heat storage systems using water or rock as the heat storing medium are commonly used. Water is an efficient heat storage medium because a small volume of water can store a relatively large amount of heat. However, heat storage systems using water as the storage medium have many problems related to pumping, heat exchangers, piping, valves, location and placement of large tanks, leaks, and insulation. The insulation problems are magnified by the presence of thermal convection currents in large water storage tanks.
Rock piles do not have leakage problems or large convection currents. They heat air directly and thereby eliminate heat exchanger systems. However, rock storage systems are bulky inasmuch as rocks require roughly three times the volume of water to store the same amount of heat. Some units, for ordinary home space-heating use can require as much as 42,000 lbs. of rock. Moreover, such large amounts of rock are commonly housed in structures which are relatively large and thereby occupy valuable ground space. For the majority of present day heat storage applications, neither a water system nor a rock system provides a good, viable solution to heat storage for solar energy systems.
A minimum volume occupied by the heat storage system is an important aspect to be considered in selecting a heat storage system. The solar heat engineer or architect would like to store the maximum amount of heat energy in a minimum volume. In this context, a unit of measurement called the "storage figure of merit (SFM)" with units of BTU/cu.ft./.degree.F., is significant. SFM for a given material is measured by the product of the material's specific heat and its density. For example:
______________________________________ MATERIAL SP.HEAT DENSITY SFM ______________________________________ Water 1.0 62.4 62.4 Granite (Rock) 0.2 170 34.0 Dry Gravel 0.2 120 24.0 Stainless Steel 0.11 487 53.57 Copper 0.1 540 54.0 Aluminum 0.2 165 33.0 ______________________________________
Of the above materials, water has the highest SFM, but heat storage systems using large tanks of water have the problems described above. Stainless steel, copper and aluminum are good heat storage mediums, but the cost of these materials is prohibitive if used largely in a heat storage system. Granite has only slightly more than half the SFM of water, but its low cost makes it competitive for heat storage applications because a larger volume is all that is required for storing more heat, and such a system does not require heat exchangers, complex plumbing, or the installation required by water systems.
Another factor to consider in selecting a heat storage system is the time required to reheat the storage system after some of the heat energy has been withdrawn for constructive use, such as in heating a home. For a water system the reheating time is relatively long because the entire volume of water, typically in the 1500 gallon range for home heating uses, must be heated as a single mass. In a rock system, the reheating time is relatively long because rock is a poor conductor of heat and therefore it requires a relatively long time to conduct heat to the center of each rock.
Thus, there is a need to provide a heat storage system having a relatively high SFM, minimal leakage problems; small, if any, thermal conduction current problems; slow heat loss to the surface (low internal thermal conductivity) to reduce insulation requirements; fast reheating time; and low cost.